The present invention relates generally to a method of manufacturing a powdery AlN, and more particularly to a combustion synthesis method of manufacturing a powdery AlN. Having a high thermal conductivity, a high electrical resistivity, a good mechanical strength, and a good oxidation and thermal-shock resistance, AlN becomes a very important ceramic material in industrial applications. It can be used for high-performance electronic substrate material, optical lenses, cutting tools, heat sinks, and many high-temperature structure materials.
The manufacturing methods for AlN include:
1) the gas phase reaction method, e.g., ##STR1##
2) the direct nitridation method, e.g., ##STR2##
3) the reduction-nitridation method, e.g., ##STR3##
4) the combustion synthesis method.
The gas reaction method is not suitable for mass production of AlN in industry because of the high cost and slow manufacturing rate involved.
The direct nitridation of Al and the nitridation of powdery Al.sub.2 O.sub.3 methods require a process executed under a high temperature and a long period of time, e.g., 5 hours, to fully complete the reaction, which can thus result in common disadvantages including a greater energy consumption and a slow manufacturing rate.
In comparison to other methods, the combustion synthesis method is a new method used to synthesize ceramic materials by self-propagation combustion reactions. The advantages achieved thereby include that it has a fast reaction rate, a less energy consumption and a simple manufacturing process and that it can be used for mass production. Several combustion synthesis examples are as follows:
1) The Japanese Patent Publication NO. 63-274605: Al, AlN and CaCO.sub.3 (or other additives) powders, in an appropriate ratio, are mixed and then pressed into pellets. The pellets are ignited by electrical heating under 50 atm N.sub.2 gas to form the powdery AlN.
2) The Japanese Patent Publication No. 64-76906: Al and powdery AlN in an appropriate ratio, are mixed and placed in a porous, refractory vessel surrounded by liquid nitrogen. The synthesis reaction is ignited by electrical heating to form powdery AlN.
3) The Japanese Patent Publication No. 64-76905: Powdery Al and NaN.sub.3 (or other solid-state nitrides, e.g., KN.sub.3, Ba.sub.3 N.sub.2 . . . etc.) in an appropriate ratio, are mixed and put into a refractory vessel. An igniting agent is placed on the top of the powder mixture. The vessel is then placed in an electrical oven which is enclosed in a container filled with N.sub.2 gas (having a pressure smaller than 10 kg/cm.sup.2). The powdery AlN can thus be formed by first heating the powdery Al and NaN.sub.3 in the oven and then igniting the igniting agent to start and complete the combustion synthesis reaction. The above combustion synthesis technology for the production of powdery AlN still has two key problems to be overcome, i.e.
1) How to supply sufficient nitrogen and to have it mixed thoroughly with the powdery Al; and PA1 2) How to fully complete the reaction. PA1 1) It utilizes powdery Al and solid-state nitrides: The present invention uses powdery Al and solid-state nitrides (e.g., NaN.sub.3, KN.sub.3, Ba.sub.3 N.sub.2 . . . etc.) as the reactants. The reactant powders, in an appropriate proportion, are thoroughly mixed and then moulded into a pellet of a proper shape (e.g., a cylindrical one). During the preparation of the pellet, an appropriate amount of a diluting agent (e.g., the powdery AlN) can also be added therein. PA1 2) The pellet is wrapped up with an igniting agent: The pellet prepared in the previous step is completely wrapped up with a proper igniting agent (e.g., the mixed powdery Ti+C or Al+Fe.sub.3 O.sub.4). This step can be accomplished by putting the pellet into a mold of the proper size and shape, then pressurizedly covering the pellet with the powdery igniting agent. Before the pellet is wrapped up with the igniting agent, it can firstly be wrapped up with a metal foil (e.g., Al foil), or placed in a porous refractory vessel. PA1 3) Its reaction is executed under a low pressure of N.sub.2 : The reactant pellet wrapped up with the igniting agent is placed in a vessel filled with N.sub.2 gas. The igniting agent is ignited by heating with a proper heating element (e.g., tungsten wire, graphite ribbon, or a laser) by which the igniting agent can be heated to have a temperature up to 800.degree.-1700.degree. C. resulting in ignition and then triggering the combustion of the reactant pellet, thus forming the high quality powdery AlN. Although such reaction can take place in N.sub.2 gas of a higher pressure, it only requires an N.sub.2 gas pressure of 1-7 atm. The most notable differences between the present invention and the conventional combustion method of manufacturing powdery AlN are that in the present invention, the reactant pellet is wrapped up with an igniting agent, and the reactant pellet is enveloped in the igniting agent after encapsulated in a metal foil or placed in a porous refractory vessel. Wrapping the reactant pellet with an igniting agent requires that the reactant pellet be directly and completely wrapped up with an igniting agent, or the reactant pellet be first covered with a metal foil or be put into a porous refractory vessel and then wrapped up completely with an igniting agent.
According to the reported study (such as M. Costantino and C. Firpo, J. Mater. Res. 6:2397(1991)), if the N.sub.2 gas is used as the nitrogen source, the pressure thereof must be greater than 1000 atm to start the reaction.
The aforementioned Japanese Patent Publication No. 63-274605 uses 50 atm N.sub.2 gas which is still a relatively high pressure. Such high pressure of N.sub.2 gas will not only result in a higher cost on apparatus and operation, but in more complexity and danger during the operation as well.
If liquid nitrogen is used as the nitrogen source (such as the Japanese Patent Publication No. 64-76906), the high pressure of N.sub.2 gas is not necessary. The low temperature of the liquid nitrogen, however, will not only result in a higher cost on apparatus and operation, but in more complexity and danger during the operation as well.
If a solid-state nitride is used as the nitrogen source (e.g., in the Japanese Patent Publication No. 64-76905), the high pressure of N.sub.2 gas is also not necessary. However, in order that the reaction can proceed in a self-propagation combustion manner, the solid-state nitride should be an easily thermally decomposable compound and the reaction must be well-controlled so that the nitrogen produced by the decomposition can rapidly react with the powdery Al. Otherwise, there will be a resulting high pressure and the reaction is no more operative owing to the fact that the N.sub.2 gas is escaping.